1. Field of the Invention
The present invention relates generally to metal oxide varistors and, more particularly, to a metal oxide varistor that can modify its operating characteristics to protect a ground fault circuit interrupter during the occurrence of an overload voltage surge.
2. Description of the Prior Art
A high voltage transient surge can totally or partially damage electrical devices such as Ground Fault Circuit Interrupters (GFCIs) located in homes, factories and commercial buildings. In many instances the damage can cause only the protective features of the GFCIs to become either partially or fully inoperative while the device itself continues to conduct electricity. For example, it is not uncommon for the contacts of a GFCI which was subjected to a high voltage transient surge to be fused together and continue to conduct current even while the protective features of the GFCI are no longer operational.
A need exists for a device which can protect loads from short term over-voltage conditions. One class of devices which can be used to protect the GFCI from an over-voltage condition is known as Metal Oxide Varistors (MOVs). In operation, an MOV is connected in parallel with the device that is to be protected such as a GFCI. At low voltages the MOV has a very high resistance. At high voltages, the varistor has a very low resistance so that when a high voltage transient surge appears on the power supply line, the MOV, which appears as a low resistance, prevents the transient voltage surge from reaching the device. Conduction through an MOV begins when the voltage across the MOV reaches a maximum continuous operating voltage, referred to as the varistor voltage. As the voltage increases, the MOV's resistance drops rapidly and may approach zero. Because the resistance of the MOV decreases as the voltage increases, the MOV diverts transient current through itself and not through the device that is connected in parallel with and up stream of the MOV. After the occurrence of the voltage transient surge, the MOV returns to its normal high resistance state and is ready for the next high voltage surge.
Another characteristic of an MOV is that during operation, the MOV will increase in temperature as it conducts high voltage surges. If the voltage surges are well spaced, the MOV can cool down between events. However, if the events are closely spaced, the MOV will not have enough time to cool down and this heating of the MOV will allow additional current to flow through the MOV. The additional current will further raise the temperature of the MOV, and this will continue until the MOV destroys itself. This condition is known as thermal runaway. When in its thermal runway state, an MOV can explode and possibly cause extensive damage to surrounding components, a fire hazard and/or injury.
One way of protecting the MOV itself is with a thermal protection device wired in parallel with and located to be heated by the MOV element. The melting point of the thermal protection device is set to be at a temperature below that which will cause the MOV to enter its thermal runaway state. As the temperature of the MOV rises, a point will be reached where the thermal protection device will melt and disconnect the MOV from the load. When the load is a GFCI, it will no longer be protected by the MOV and the full impact of the high voltage transient pulse will be applied to the GFCI. Thus, when an overload condition occurs, the over voltage transient surge is free to destroy the GFCI that was being protected.
What is needed is an MOV which can protect a GFCI during an overload voltage surge.
The peak surge current rating of an MOV is a function of the area of the disc itself. To protect a GFCI from destructive high voltage transient surges, test have shown that an MOV of at least 20 mm is needed. Unfortunately, it is not possible to connect an MOV of this size to a GFCI and still fit the GFCI and the MOV into a single outlet box.
What is also needed is an MOV which, when connected to a GFCI, is small enough to fit within a single outlet box.